Approximately 80 million years ago, dinosaurs roamed much of North America.  Although it would still be another 12 million years before Tyrannosaurus rex (T. rex) emerged as one of the largest apex predators of the dinosaur kingdom, other tyrannosaurs were just starting to evolve.  Tyrannosaurs were just one of many types of therapods:  two-legged carnivores such as T. rex and Velociraptor that likely paved the way for birds.

When an 80-million-year-old therapod fossil was found in the San Juan Islands in April 2012 by a group of researchers from the Burke Museum of Natural History and Culture in Seattle, it was the first dinosaur fossil discovered in Washington state.  The fossil was classified as a piece of the therapod's left femur—about 17 inches of what scientists estimate would have been a 4-foot-long leg bone.  The fossil was deposited in the Cedar District Formation (Formation).  Scientists disagree on where this Formation was located when the therapod died; estimates range from Baja California, Mexico, to Northern California.

In any event, the therapod fossil likely immigrated to present-day Washington as the Formation traveled north tens-of-millions of years ago.  The rock eventually became part of Sucia Island, an island in the San Juan Islands that today houses a state park accessible only by watercraft.

In the first academic paper describing the fossil, University of Washington paleontologist Christian Sidor and then-PhD student Brandon Peecook hypothesized that the therapod was likely a tyrannosaur, and they included images comparing the femur to that of a Daspletosaurus.  As scientists lack enough of the therapod's bones to classify the species definitively, the therapod to which the femur fossil belonged has been nicknamed the Suciasaurus rex in recognition of the island where it was discovered.

Dinosaur fossils are rare in the Pacific Northwest because active tectonic plate boundaries nearby have resulted in significant geological turmoil.  In addition, the dense development of cities in Western Washington has curtailed scientists' ability to dig for fossils.  Washington has previously designated the Columbian mammoth as the state fossil.  According to many scientists, the state bird—the willow (or American) goldfinch—is also considered a type of theropod.  Twelve states and Washington D.C. have official state dinosaurs.

The Suciasaurus rex is the state dinosaur of Washington.